Relaxation oscillator



May 12, 1942. H. PIEPLOW 2,282,340

BELAXAIION OSCILLATOR Filed June 10, 1939 A. lAA II INVENTOR HA NSWERMZ. P/EPL 0W ATTO R N EY Patented May 12 1942 Banswerner lieplow,Berlin, Germany, winner to General Electric Company, a corporation ofNew York Application June 10, 1939, Serial No. 278,546 In Germany June23, 1938 4 Claims. (Cl. 250-36) ment, this figure being referred to as abasis for explaining the improvements which constitute the invention,

Fig. 2 is a circuit diagram of the improved relaxation oscillator in itssimplest form, and

Figs. 3 to 6 inclusive show progressively more complex circuitarrangements by which the invention may be carried out.

A known circuit is shown in Fig. l. A control tube i is provided forcharging the relaxation condenser 2 which may then be discharged acrossa gaesous discharge tube 4. The grid of the gaseous discharge tube 4 hasa negative biasing potential applied thereto from the source I i. As isknown, the frequency of relaxation oscillations produced by means ofsuch circuit is limited by the finite de-ionization time of thedischarge tube 4 and lies below about 70 kilocycles. The relaxationmechanism functions in the followin manner: 7

At the beginning of the discharge a certain excess in ions is producedso that the condenser potential at 2 decreases below the staticoperating potential of the discharge tube 4; then the ions must becarried off at such speed that the new potential increase at thecondenser 2 no longer causes ionization. Hence the discharge isinterrupted. This control of the ion supply can be achieved by the knowninsertion of the outer resistor 3, and furthermore by the choice of thegrid biasing potential from the source II. This grid bias must not benegative to such a high degree that a noticeable lack in ions occurs inthe beginning of the discharge, and on the other hand, it should benegative to such an extent that at the end of the discharge the ions canbe rapidly carried off. These opposing requirements give the reason forthe relatively low limit frequency in the relaxation oscillationoperation when using gaseous discharge tubes.

According to the present invention for the purpose of increasing thislimit frequency, means are provided which, at the end of the discharge,

produce a sudden and steep rise of the negative potential of the grid ofthe discharge path such that the grid openings are closed up by spacecharge layers. The rise of the grid potential, therefore, has the effectthat the space charge layers flow together in the grid openings in anabrupt manner.

This desired behavior of gaseous discharge tubes is known as such andhas, for instance, the effect that upon sudden application of a negativegrid potential an arc can be extinguished. When utilizing thisphenomenon in the relaxation oscillation operation, the result is thatthe requirement to discharge the condenser below the static operatingpotential can be dispensed with. Then it is only necessary to take carethat the grid becomes so negative at the end of the discharge that thearc will be extinguished and the tie-ionization period which wasnecessary, ac-

cording to Fig. l, for the-entire anode-cathode space will be reduced tothe time necessary to confine the ions in the region of the grid meshalone to form space charge layers.

Such a circuit can be realized in various ways. Thus the grid of thedischarge path can be remotely controlled, for instance, by theovervoltage produced when disconnecting an inductance. However, theplate potential may also be so controlled that the pattern of the gridpotential is negative, and substantially similar to the pattern of thepotential at the relaxation condenser 2. In certain cases the resistance3 in the plate circuit of the discharge tube 4 may be omitted and thearrangement would still function in the desired manner.

Several circuits, according to the invention, are described in thefollowing in reference to Figs. 2 to 6. A common characteristic of thesecircuits is that the grid potential of the discharge tube 4 iscontrolled by the voltage of the relaxation condenser. However, it mustbe emphasized here that the particular features of the circuits may alsobe utilized to advantage where such control does not take place, suchas, for instance,

in the case where the grid is remotely controlled.

Fig. 2 shows a circuit in which the operation is carried out with thefloating cathode of the discharge tube 4 whereby the grid is connectedacross a resistance 5, which should not be too high, to the negativeterminal of the potential source I! or to another point having fixedpotential relative to the positive plates of the relaxation condenser.In an actual embodiment i 5 of this system a control tube of the pentodetype 1 was used together with a gaseous discharge tube of the typehaving single hole grid and helium filling with a pressure of about 0.6mm.

The pentode tube as shown in Fig. 2, as well i as in Figs. 3 to 6inclusive, comprises a cathode IS, an anode I4, a control grid II, ascreen grid I6, and a suppressor grid II. The grids are connected topoints of suitable potential in a conventional manner. That is to say,the control grid may be biased by means of the source I9 whichinterconnects said grid and the cathode. The screen grid is fed from thepositive terminal The condenser 2 hasa value of about 50 miicro-microfarads, the resistor 3 about 2000 ohms,

and the resistor 5 not higher than 1000 ohms.

1 With this circuit relaxation oscillations of 1 megacycle can beobtained at a charging velocity jof 400 volts per microsecond withoutamplification. As compared with the best relaxation cir- The resistor 5in the grid circuit of the dis- ;charge tube 4 should be as small aspossible, since otherwise the space charge layers cannot close in aboutthe grid with the desired rapid- ;ity; the resistor may eventually alsobe dispensed with. At low values of the grid resistance the frequencyand the voltage rise at the relaxation condenser show pronouncedsaturation phenomena, while the latter decrease in the case of highvalues. There exists likewise an optimum value as regards the size ofthe plate resistor 3. If this resistor is too large the discharge of thecondenser is too slow. Therefore, the grid (as compared with thecharging velocity) does not become negative rapidly enough, so that thecharging velocity of the condenser must be reduced by decreasing thecurrent. With. a very small plate resistor, however, the intensity ofthe ion current is too high at the end of the discharge, so that in thiscase the oscillating current should be reduced. There is an optimumvalue of the resistor 3 between these two cases, this value beingsubstantially as given above for the special case. The optimumresistance can be easily found by varying the other parameters of thecircuit. The charging tube I need not be a pentode. In place thereof itis possible to use a different tube or combination of tubes, or anyother resistor, for instance, an ohmic resistor.

* It has been suggested to place the grid tapping of the discharge tubeonly at a part of the driving potential, it being known to connect it toa part of the charging resistance (German Patent 636,059). 0n thecontrary the present invention proposes (for instance, by the connectionwith-the negative pole of the driving direct potential) to impart to thegrid such a negative poof the potential source I2 through a resistor I8.i The suppressor grid I1 is directly connected to 1 the cathode II. Thismanner of connecting the grids is well known in the art and is shown,for 1 example, in the RCA Receiving Tube Manual,- 1 Technical SeriesRG13, published 1937. Referring to page 36 and to Fig. 43 thereon, theconinections of the pentode tube to an operating potential source isthere shown.

tential that the desired space charge eifect takes place.

In such a relaxation arrangement the influx of ions into the grid of thedischarge vessel is obviously very great. Since this ion current entersin part the cathode side and in part also the anode side of the grid, anadditional charging of the relaxation condenser takes place on accountof the influx at the cathode side, which additional charging tends todestroy the advantages of a saturation tube or of any arrangement forproducing a linear potential rise: and causes a non-linear distortion ofthe condenser voltage. According to a further feature of the presentinvention, arrangements are provided which eliminate this non-lineardistortion.

Fig. 3 shows the use of a high-vacuum tube 6 series connected in thecathode circuit of the gaseous discharge'tube I. The function of thetube 6 is to suppress the ion current at the oathode side. To obtainhigh frequencies the tube 6 is designed to offer an extremely lowanodecathode capacity.

Fig. 4 shows the use oi a high-vacuum tube I inserted in the gridcircuit of the gaseous discharge tube 4. The working point of the tube Iis so set that the ion influxat the cathode side of the tube 4 ismaintained constant. In some cases it may also be desirable to operatethe tube I, especially a pentode, under a condition of saturation,whereupon the entire ion current of the grid remains constant. In thiscaseit is true that the ion current at the cathode side which influencesthe condenser charge is not exactly constant, but this error ispractically negligible if this current exceeds considerably the currentat the anode side, such as is generally the ease. The use of asaturation tube has the further advantage that the synchronizingimpulses can be applied thereto in a simple manner, for instance, to thecontrol grid at 8. 2,-

According to a further feature of the invention this ion current at theside of the cathode may also be utilized to charge the condenserintentionally as fast as possible; since the ion current generally has ahigher peak than the current passing through the controlling tube I. Itis only necessary to adjust the circuit constants so that the currentwill at first be choked -somewhat by the control tube and then releasedagain after the ion current of the grid has died down.

1 Such a requirement is satisfied, for instance, by w a circuitaccording to Fig. 5 in which the grid current passes through a cathoderesistor 9 of the control tube I.

An arrangement mayalso be used according to Fig. 6 in which the gridcurrent controls the tube I across an additional auxiliary tube I0. Suchan arrangement oiiers the double advantage, first that of a linearcondenser charging, and secondly that of a very rapid condenser chargingwhich is possible especially by utilizing the high grid ion currentwhich is at first of a detrimental nature.

Referring to-Fig. 6 in more detail, the positive ion current to the gridof tube 4, (which occurs when this tube breaks down) makes the grid oftube III more positive, hence makes the grid of tube I more negative.This action blocks the tube I. The current through tube I does not reactupon the grid-cathode potential difierence of tube I. Therefore, thereis no self-opposing or degenerative action of the tube I such as wouldwork against the control voltage applied thereto by the amplifying andphase reversing action of the tube I0. As soon as the ion current to thegrid of tube 4 ceases, the grid of tube ll assumes normal bias.Simultaneously the grid of tube i assumes such a bias as to render thistube conductive, which permits the condenser 2 to start chargin again.

Finally, although as a whole a linear voltage rise at the condenser isaimed at, the discharge tube, in this case, need not be a saturationtube since, in fact, it is not solely the electron current passingthrough this tube which is decisive. While the arrangement according toFig. 5 has the advantage of the lesser requirement, the arrangementaccording to Fig. 6 has the advantage of a more flexible control. In thearrangement according to Fig. 5 there appears on account of the ionicgrid current an additional potential drop through the resistor 9 whichrenders the control grid of tube l negative, thereby decreasing thecurrent in the charging tube as desired. But with the decrease of thiscurrent the potential drop through the resistor 9 likewise decreases,and hence the control impulses are opposed by the controlled pulses oicurrent through resistor 9. On the other hand, the resistor 5, whichshould be as small as possible to favor the rapid leading away of theions, cannot be used for the direct control of the charging tube.Therefore, the arrangement according to Fig. 6 is chosen which alsocontains in addition an amplifier tube II for eflecting the necessaryamplification oi. the impulses and also a phase reversal, so that it ispossible directly to control the grid 01' the charging tube I withoutreaction on this control by the current that is controlled. Thus thedescribed partial elimination of the control impulses by the controlledimpulses is avoided.

I claim:

1. A relaxation oscillator circuit arrangement comprising a gaseousdischarge tube and a vacuum discharge tube, each having an anode, acathode, and at least one grid, the cathode of the gaseous dischargetube being in circuit with the vacuum tube, a direct current sourcehaving its positive terminal connected through a resistor to the gaseoustube anode and its negative terminal in circuit with the vacuum tubecathode, a capacitor connected between the positive terminal of thesource and the vacuum tube anode, impedance means conecting the grid ofthe gaseous tube to the negative terminal of said source, and means forcontrolling the potential of the grid of said vacuum tube in accordancewith the potential across said impedance means.

2. A circuit arrangement according to claim 1 wherein said impedancemeans is a resistor.

3. A circuit arrangement according to claim 1 wherein said lastmentioned means comprises a third discharge tube, the input of which iscoupled to said impedance means and the output of. which is coupled tothe grid ofsaid vacuum tube.

4. A relaxation oscillator circuit arrangement comprising a gaseousdischarge tube, a vacuum discharge tube and an auxiliary control tube,each said tube having an anode, acathode and at least one grid, a directcurrent source having its positive terminal connected through a resistorto the gaseous tube anode, and its negative terminal in circuit with thevacuum tube cathode, a connection from the vacuum tube anode to thegaseous tube cathode, a capacitor coupled between said connection andthe positive terminal of said source, means for coupling the anode ofsaid auxiliary tube to the grid of said vacuum tube, a direct connectionbetween the grid of said gaseous tube and the grid of said auxiliarytube, an impedance connected between the grid of said gaseous tube andthe negative terminal of said source and'a connection from the cathodeof said auxiliary tube to said impedance whereby the potential of thegrid of said vacuum tube is controlled in accordance with the potentialacross said impedance.

HANSWERNER PIEPLOW.

